Development of new fabrication technologies for silicon-based microstructures is currently under aggressive pursuit. Silicon-based microstructures commonly comprise silicon and silicon oxides, but can also comprise silicon nitride, metal thin films, and a range of other structural materials known in the art. Such microstructures are commonly fabricated using a range of techniques originally developed for fabrication of silicon integrated circuits, and find application in the area of microelectromechanical systems (MEMS), and in microfluidic systems, such as micro-scale chemical sensors, gas chromatographs, and the like.
It is held desirable in the art to fabricate a desired microstructure so that the number of gross assembly steps is limited. Gross assembly steps are those which provide external functional interconnection between individually fabricated subassemblies. Gross assembly steps require precision handling, positioning, and bonding of delicate millimeter-scale structures, which are fraught with opportunities for failure. In particular, when the goal is a micromechanical device, the bonding materials can easily interfere with the desired motion of the components of the device. Similarly, when the goal is a microfluidic device, the bonding materials can block the flow channels, or prevent microvalves from functioning properly.
Working against the above motivations for direct fabrication of complex microstructures is the difficulty of fabricating such microstructures in their entirety. As the complexity of a process required for fabrication of a desired silicon microstructure increases, the useful yield of that process falls rapidly toward zero. Complex fabrication processes are not only susceptible to error, but in many cases can lead to accumulation of residual stress, nonplanarity, and other structural flaws which can render the desired product unusable.
The etching of features into a silicon-based microstructure plays a central role in the fabrication of micro-scale silicon-based devices. Both wet etching and dry etching are commonly used in such fabrication. Wet etching of silicon is commonly carried out using liquid etchants including, but not limited to, KOH, tetramethyl ammonium hydroxide, or ethylene diamine pyrochatechol. In contrast, dry etching generally comprises the application of reactive neutrals and ionic etchants generated in a plasma to the surface to be etched. Of particular utility in silicon-based microstructural fabrication is deep reactive ion etching.
Both wet and dry etching procedures exhibit an effect called aspect-ratio-dependent etching, in which a narrow feature being defined by etching deepens less rapidly than does a wide feature being defined under the same etching conditions. In other words, the etching rate varies with the size of the feature being defined. Aspect-ratio-dependent etching effects can result in the need to define and etch narrow features separately from wider features, resulting in an undesirable increase in the complexity of the overall fabrication process.
There is an ongoing need in the art for microstructural fabrication techniques which allow the development of less complex fabrication processes.
The present invention addresses this need by enabling increased control over the definition by etching of structural features having widely different pattern widths.
These and other advantages of the method of the present invention will become evident to those skilled in the art.